This invention pertains to high performance cross-linked polybenzoxazole and polybenzothiazole polymer membranes and methods for making and using these membranes.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and high selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
Polymers provide a range of properties including low cost, good permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through glassy polymers more slowly as compared to polymers with less stiff backbones. However, polymers which are more permeable are generally less selective than less permeable polymers. A general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). Over the past 30 years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success. In addition, traditional polymer membranes also have limitations in terms of thermal stability and contaminant resistance.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used commercially for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. It has been found that polymer membrane performance can deteriorate quickly. The primary cause of loss of membrane performance is liquid condensation on the membrane surface. Condensation can be prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes. Although these pretreatment systems can effectively perform this function, the cost is quite significant. In some projects, the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the pretreatment system cost or total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. On the other hand, in recent years, more and more membrane systems have been applied to large offshore natural gas upgrading projects. For offshore projects, the footprint is a big constraint. Hence, reduction of footprint is very important for offshore projects. The footprint of the pretreatment system is also very high at more than 10 to 50% of the footprint of the whole membrane system. Removal of the pretreatment system from the membrane system has great economic impact, especially to offshore projects.
High-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole have been developed to improve membrane selectivity, permeability, and thermal stability. These polymeric membrane materials have shown promising properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8). However, current polymeric membrane materials have reached a limit in their productivity-selectivity trade-off relationship. In addition, gas separation processes based on the use of glassy solution-diffusion membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer as represented by the membrane structure swelling and significant increases in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
Aromatic polybenzoxazoles (PBOs), polybenzothiazoles (PBTs), and polybenzimidazoles (PBIs) are highly thermally stable ladderlike glassy polymers with flat, stiff, rigid-rod phenylene-heterocyclic ring units. The stiff, rigid ring units in such polymers pack efficiently, leaving very small penetrant-accessible free volume elements that are desirable for polymer membranes with both high permeability and high selectivity. These aromatic PBO, PBT, and PBI polymers with high thermal and chemical stability, however, have poor solubility in common organic solvents, preventing them from being used as common polymer materials for making polymer membranes by the most practical solvent casting method. Thermal conversion of soluble aromatic polyimides containing pendent functional groups ortho to the heterocyclic imide nitrogen in the polymer backbone to aromatic polybenzoxazoles (PBOs) or polybenzothiazoles (PBTs) could provide an alternative method for creating PBO or PBT polymer membranes that are difficult or impossible to obtain directly from PBO or PBT polymers by solvent casting method. (Tullos et al, MACROMOLECULES, 32, 3598 (1999))
On the other hand, some inorganic molecular sieve membranes such as SAPO-34 and carbon molecular sieve membranes offer much higher permeability and selectivity than polymeric membranes for gas separations, but are high cost, have poor mechanical stability, and are difficult for large-scale manufacture. Therefore, it is still highly desirable to provide an alternate cost-effective membrane with improved separation properties.
In U.S. Pat. No. 5,409,524, a number of different polymer membranes were treated by heating the membrane to relax excess free volume in the polymer. This would tend to decrease the permeability of the polymer. The heating of the membrane was in a temperature range from 60 to 300° C. The membranes were then irradiated with UV radiation in the presence of oxygen to at least partially oxidize the surface. One of the membranes that were reported treated was a polybenzoxazole membrane. The polybenzoxazole polymer was prepared by a one-step polycondensation synthesis procedure and the membrane prepared from this polybenzoxazole polymer was heat treated at about 180° C. The membrane films exhibited about a 25% decrease in permeability from 12.25 Barrer to 9.11 Barrer and about a 15% increase in oxygen/nitrogen selectivity from 5.34 to 6.21. These conditions produced a minor increase in selectivity compared to the present invention which used different starting materials as well as a significantly higher membrane treating temperature.
A recent publication in the journal SCIENCE reported a new type of high permeability polybenzoxazole polymer membranes for gas separations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). These polybenzoxazole membranes are prepared from high temperature thermal rearrangement of hydroxy-containing polyimide polymer membranes containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen. These polybenzoxazole polymer membranes exhibited extremely high CO2 permeability (>1000 Barrer) which is about 100 times better than conventional polymer membranes and similar to that of some inorganic molecular sieve membranes but the CO2/CH4 selectivity was similar to commercial cellulose acetate membranes. Improved selectivity is needed for these membranes to be of commercial use. The authors tried to increase the selectivity of these polybenzoxazole polymer membranes by adding small acidic dopants (e.g., HCl and H3PO4). However, the stability of the small acidic dopants in these polybenzoxazole polymer membranes is a critical issue for commercial use.
In US 2008/0300336 A1, it was reported that the use of UV crosslinking did succeed in improving the properties of certain mixed matrix membranes that contain molecular sieves that function to improve the permeability and selectivity of the membranes. However, it was necessary to both crosslink the polymer and to add the molecular sieves to obtain the improved levels of performance reported therein. It is highly desired to have improved polymeric membranes that do not contain molecular sieves both to avoid the need to disperse the molecular sieves and to eliminate any problems caused by the lack of adhesion between the polymer and the molecular sieves.
The present invention overcomes the problems of both the prior art polymer membranes and inorganic molecular sieve membranes by providing a new type of high performance cross-linked polybenzoxazole and polybenzothiazole polymer membranes and a route to make said membranes that have the following properties/advantages: ease of processability, both high selectivity and high permeation rate or flux, high thermal stability, and stable flux and sustained selectivity over time by resistance to solvent swelling, plasticization and hydrocarbon contaminants. These membranes provide much better permeability when compared to crosslinked polyimide membranes and much better selectivity when compared to uncrosslinked polybenzoxazole membranes.